Method of measuring blood pressure with a plethysmograph

ABSTRACT

A method and apparatus for determining arterial blood pressures using a photoplethysmograph is provided. In the inventive method the photoplethysmograph output is calibrated to the patient using an auxiliary blood pressure determining instrument, a constant k particular to the patient&#39;s arterial blood pressure-volume relationship is determined and is stored for later use; at the time of measurement information is obtained from the photoplethysmograph output; and a computer determines the patient&#39;s systolic and diastolic blood pressures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.07/656,021 filed Feb. 15, 1991, now U.S. Pat. No. 5,140,990, which is acontinuation of U.S. application Ser. No. 07/579,159 filed Sep. 6, 1990,now abandoned.

DESCRIPTION

1. Technical Field

This invention relates generally to blood pressure measurements. Moreparticularly, it relates to a method of non-invasively determining bloodpressure using a photoplethysmograph.

2. Background of the Invention

Arterial blood pressure measurements provide valuable information abouta patient's condition. The heart's cyclical action produces a bloodpressure maximum at systole, called systolic pressure, and a minimumpressure at diastole, called diastolic pressure. While the systolic anddiastolic pressures are themselves important in gauging the patient'scondition, other useful parameters are the mean (average) blood pressureduring a heart cycle, and the pulse pressure, which is the arithmeticdifference between the systolic and diastolic pressures.

The importance of arterial blood pressure has spurred the development ofnumerous methods of determining it. The most widely used method isprobably the familiar blood pressure cuff, which consists of anexpandable ring (1) inflated to stop arterial blood flow and (2) thengradually contracted. Using a stethoscope, medical personnel listen tothe artery to determine at what pressure blood flow begins, establishingthe systolic pressure, and at what pressure flow is unrestricted,establishing the diastolic pressure. More advanced blood pressuremonitoring systems plot the arterial blood pressure through a completeheart cycle. Typically, these systems use catheters having piezoelectricpressure transducers that produce output signals dependent upon theinstantaneous blood pressure. The output signals are monitored and usedto determine the arterial blood pressures over a complete heart cycle.These systems are advantageous in that the blood pressure iscontinuously measured and displayed.

While prior art methods are useful, they have disadvantages. Cuff-typesystems require restricting arterial blood flow and are not suitable forcontinuous use. The piezoelectric-type systems generally requireundersirable invasive techniques, costly disposable materials, and timeand skill to set-up. However, during certain critical periods, such assurgery, continuous arterial blood pressure monitoring is highlydesirable. Therefore, it would be beneficial to have a method ofcontinuously and non-invasively measuring a patient's blood pressure.

Photoplethysmographs are well-known instruments which use light fordetermining and registering variations in a patient's blood volume. Theycan instantaneously track arterial blood volume changes during thecardiac cycle. Since photoplethysmographs operate non-invasively, muchwork has gone into using them to determine blood pressure. In 1983,inventor Warner was issued U.S. Pat. No. 4,418,700 on a method ofdetermining circulatory parameters, wherein signals from aphotoplethysmograph were used to determine arterial blood pressure.

Significant problems were found when investigating the Warner method.Therefore, it is clear that the need for a practical method ofcontinuously and non-invasively monitoring arterial blood pressure hasremained.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved method forcontinuously and non-invasively measuring arterial blood pressure.

It is another object of the present invention to provide an improvedmethod and system for non-invasively determining arterial systolic anddiastolic blood pressures with a photoplethysmograph.

These and other objects, which will become apparent as the invention ismore fully described below, are obtained by providing a method andapparatus for determining arterial blood pressures using aphotoplethysmograph. The inventive method comprises the steps ofcalibrating the photoplethysmograph output with a patient's arterialblood pressure to determine an arterial constant k in the formula,

    ψ=ψ.sub.inf (1-Kexp(-kP))

where ψ is the arterial blood volume, ψ_(inf) is a conversion constantcorresponding to arterial blood volume at infinite pressure, K and k arearterial constants for the patient, and P is the instantaneous arterialblood pressure; gathering data from the photoplethysmograph outputduring a measurement period; and computing the arterial systolic anddiastolic pressures at the measurement period using the evaluatedarterial constant k and the data gathered during the measurement period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cutaway view, partial application depiction, andpartial block diagram illustrating a preferred method in operation.

FIG. 2 is a sketch of the output waveform from a photoplethysmographreceiver over two cardiac cycles.

FIG. 3 is a block diagram illustrating the basic procedural steps of thepreferred method of FIG. 1.

FIG. 4 is a flow diagram of the preferred procedure for calibrating thephotoplethysmograph output to a patient according to the inventivemethod.

FIG. 5 is a flow diagram of the output monitoring and data acquisitionsteps according of the inventive method.

FIG. 6 is a flow diagram outlining the preferred procedural steps forarterial blood pressure determination according to the inventive method.

FIG. 7 is a flow diagram of an alternative procedure for calibrating thephotoplethysmograph output to a patient according to the inventivemethod.

FIG. 8 is a flow diagram outlining alternative procedural steps forarterial blood pressure determination according to the inventive method.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention, shown in FIG. 1, uses atransmitter 2 portion of a photoplethysmograph 4 to cause monochromaticlight 6, preferably in the red and IR ranges, to be emitted from aphotodiode light source 8. The emitted monochromatic light 6 travelsthrough a patient 9, along a light path which includes blood 10 in anartery 12, to a photodiode light detector 14. While artery 12 has beendescribed, and is shown in FIG. 1, as a single artery, in all practicalcases the light path actually passes through many arteries. Thesearteries can be lumped together and treated as if only one artery 12existed. Therefore, for simplicity, the remainder of this applicationwill only discuss one artery 12, but it is to be understood that itrepresents the composite effects of many individual arteries. The lightpath is also through background tissue 16. The transmitter 2 controlsthe amount of monochromatic light 6 emitted by varying the amount ofcurrent through the light source 8. In the preferred embodiment, thetransmitter 2 regulates the monochromatic light 6 at a fixed level.

As the monochromatic light 6 travels along its light path it ispartially absorbed by the background tissue 16 and the blood 10. Aportion of the monochromatic light 6 is not absorbed and impinges on thelight detector 14, creating electrical signals which are applied to areceiver 18 of the photoplethysmograph 4. The magnitudes of theseelectrical signals depend upon the amount of monochromatic light emittedby the light source 8, the path lengths through the background tissue 16and the blood 10, the amount of light absorbed per unit length by theblood 10 and tissue 16, the conversion efficiency of the light detector14, and various lumped losses such as poor focusing of the monochromaticlight 6.

Since the artery 12 is pliant, as blood pressure increase so does thevolume of blood 10 within the artery 12. As the heart beats, itscyclical action causes the arterial blood pressure to change. Thiscauses the electrical signals to change since the path length throughthe blood 10 changes, causing the amount of monochromatic light 6absorbed by the blood 10 to change. Therefore, the electrical signalsfrom the light detector 14 applied to the receiver 18 is a function ofthe arterial blood pressure.

The receiver 18 amplifies the electrical signals to a usable level andapplies them as analog signals, via a receiver line 22, to ananalog-to-digital converter A/D 23. The A/D 23 converts the outputs ofthe receiver 18 to time sampled digital signals which are applied to thecomputer 24 via a computer bus 25.

The signals on the receiver line 22 can be represented by thephotoplethysmograph output waveform 26, shown in FIG. 2 for two cardiaccycles. The horizontal axis designates time and, in the presentapparatus, the vertical axis designates volts, but current levels wouldalso be suitable. Times t0 and t1, denoting the beginning of eachcardiac cycle, are clearly marked. The waveform 26 can be describedmathematically as a function of time, with the description being f(t).The voltage waveform is inverted from the common pressure waveformbecause the voltage corresponds to transmitted light. The highestvoltage obtained over a cardiac cycle, V_(d), coincides with thediastolic pressure and the lowest voltage, V_(s), coincides with thesystolic pressure. Between V_(s) and V_(d) is a mean pressure voltageV_(m), which corresponds to the mean, or average, arterial pressure overa full cardiac cycle. The duration of the cardiac cycle, t_(d) is thetime between reoccurrences of the diastolic or systolic voltages. Thearea between the waveform function f(t) and the diastolic voltage line,shown in crosshatch in FIG. 2, is called the "ARC." The particularvalues for V_(s), V_(m), V_(d), as well as the waveform function f(t)and the area ARC, change with different patients, photoplethysmographs,sensor locations, and photoplethysmograph settings. However, theseparameters are functions of the arterial blood pressure.

In a preferred method of the present invention, three major steps areused to determine arterial blood pressure, shown in FIG. 3. The first,shown in block 310, is the calibration of the photoplethysmograph outputto the patient. Referring now to FIG. 1, the calibration is accomplishedby matching the photoplethysmograph output on the computer bus 25 at thetime of calibration with the systolic, P_(s), and diastolic P_(d), bloodpressures from the auxiliary blood pressure instrument 20. In thepreferred embodiment, these blood pressure measurements are entered viaa keyboard to the computer 24. However, preferably this informationwould be entered directly via an instrument bus 28. Thephotoplethysmograph output is compared with the systolic and diastolicpressures, P_(s) and P_(d), from the auxiliary blood pressure instrument20 and several constants are determined, as is subsequently discussed.

As is shown in FIG. 3, block 320, the next step is the measurement ofthe photoplethysmograph outputs during a measurement period to determinevarious information. This information includes the systolic, mean, anddiastolic photoplethysmograph voltages V_(s), V_(m), and V_(d),respectively, the cardiac duration t_(d), and the ARC. The final steps,shown in FIG. 3, blocks 330 and 340, are the calculations of thesystolic and diastolic blood pressures, P_(s) and P_(d), respectively,using the determined photoplethysmograph information and the constantsdetermined in blocks 320 and 310. After the systolic and diastolic bloodpressures are determined, the information is output to medical personnelon a display 30. If more measurements are desired, decision block 350causes blocks 320, 330, and 340 to be repeated. However, only onecalibration phase 310 is required. These major steps are expanded uponbelow.

DERIVATION OF THE MATHEMATICAL MODEL

The principle of the inventive method is derived from the Beer-Lambertlaw of analytical chemistry. The Beer-Lambert law gives the relationshipbetween the absorption of monochromatic light by a concentration of amaterial in a solution as a function of the path length through thesolution. Mathematically, the Beer-Lambert law is expressed as:

    I=I.sub.o exp.sup.-cex

where I is the intensity of transmitted light, I_(o) is the intensity ofincident light, c is the concentration of material, e is the extinctioncoefficient of monochromatic light at a wavelength λ, and x is the lightpath length through the medium.

The present invention analogizes blood 10 and tissue 16 density toconcentration, modifies the Beer-Lambert law so that the light intensityterms are given in terms of receiver 18 output voltages, and breaks thelight path into individual lengths containing the background tissues 16and the arterial blood 10. Therefore, the modified version of theBeer-Lambert law is:

    V=ZI.sub.o exp.sup.(.sup.-c t.sup.e t.sup.x t.sup.-c a.sup.e a.sup.x a)

where the _(t) refers to the background tissues 16, _(a) refers to theblood 10 in the artery 12, V is an equivalent transmission voltagecorresponding to the transmitted light, and Z is a constant relatinglight intensity to the receiver 18 output voltage.

This can be simplified to:

    V=A.sub.o exp.sup.(.sup.-c t.sup.e t.sup.x t).sub.exp .sup.(.sup.-c a.sup.e a.sup.x a)

where A_(o) =ZI_(o).

This version has separable components, A_(o) exp.sup.(-c t^(e) t^(x) t)which relates to the conversion constant and the background tissues 16,and exp(^(-c) a^(e) a^(x) a), which relates to the arterial blood 10.For simplicity, the first component can be given as V_(o) =A₀exp.sup.(-c t^(e) t^(x) t), the background transmission voltage.Therefore, the equivalent transmission voltage can be calculated as:

    V=V.sub.o exp(.sup.-c a.sup.e a.sup.x a)

It is convenient to express the above formula in terms of arterial bloodvolume rather than light path length. Therefore, letting ψ be thearterial blood volume, and substituting for the light path ^(x) _(a) :

    V=V.sub.o exp(-bψ.sup.1/2),

where b is equal to c_(a) e_(a) (4/πL)^(1/2), and L is the light pathwidth through the artery 12. Taking the natural logarithm results in:

    1nV=-bψ.sup.1/2 +1nV.sub.o

This version becomes more useful after incorporation of the arterialvolume-pressure relationship:

    ψ=ψ.sub.inf (1-Kexp(-kP))

where ψ is still the arterial blood volume, ψ_(inf) is a conversionconstant corresponding to the blood volume at infinite blood pressure,and K and k are constants for the artery 12, and P is the instantaneousarterial blood pressure. This arterial volume-pressure relationship is agood approximation at the pressures of interest. Substituting thisformula for ψ in the logarithmic version:

    1nV=-b(ψ.sub.inf).sup.1/2 (1-Kexp(-kP)).sup.1/2 +1nV.sub.o

This can be expanded using a Taylor series. Expanding and eliminatinghigher terms results in:

    1nV=f+(n)exp(-kP)

where f is equal to 1nV_(o) -b(ψ_(inf))^(1/2), and n is equal to(Kb(ψ_(inf))^(1/2))/2. This can be converted to:

    V=(u)exp((n)exp(-kP))

where u is equal to exp(f). In terms of systolic, diastolic, and meanpressures:

V_(s) =(u)exp((n)exp(-kP_(s))) for systolic Pressure

V_(d) =(u)exp((n)exp(-kP_(d))) for diastolic Pressure

V_(m) =(u)exp((n)exp(-kP_(m))) for mean Pressure

V_(inf) =u

V_(O) =(u)exp(n)

V_(O) /V_(inf) =exp(n) Where V_(inf) is the equivalent receiver voltageat infinite pressure and V₀ is the equivalent receiver voltage at zeropressure.

Establishing various ratios:

V_(d) /V_(s) =exp((n) (exp(-kP_(d))-exp(-kP_(s))))

V_(d) /V_(m) =exp((n) (exp(-kP_(d))-exp(-kP_(m))))

ln(V_(d) /V_(s))=(n) (exp(-kP_(d))-exp(-kP_(s)))

ln(V_(d) /V_(m))=(n) (exp(-kP_(d))-exp(-kP_(m)))

and

ln(V₀ /V_(inf))=n

leads to useful ratios: ##EQU1## where P_(p) is termed "pulse pressure"and is equal to P_(s) -P_(d).

DETAILS OF THE PREFERRED METHOD

The previous section derived various relationships useful in thepreferred method as outlined in FIG. 3. The step of calibrating thephotoplethysmograph outputs to the patient 9, shown in FIG. 3, block 310is shown in expanded detail in FIG. 4. The first two steps, shown inblock 410 and block 420 are the determination and entering of thesystolic and diastolic blood pressures, P_(s) and P_(d), respectively,at calibration into the computer 24. As previously indicated and asshown in FIG. 1, these blood pressures are determined by an auxiliaryblood pressure instrument 20, preferably an accurate blood pressure cuffhaving direct inputs to the computer 24 via the instrument bus 28.

The next two steps, shown in blocks 430 and 440 of FIG. 4 are thedetermination of the photophethysmograph voltages, V_(s) and V_(d), fromthe receiver 18 output as the calibration systolic and diastolic bloodpressures, respectively. These photoplethysmograph voltages are readilydetermined since they are the minimum and maximum output signals,respectively, from the A/D converter 23. Next, as shown in block 450,the duration of the cardiac cycle, t_(d) is determined from the outputof the A/D converter 23. This is also readily accomplished by using acounter to determine the time between the diastolic voltages, times t₀and t₁ of FIG. 2.

To determine various patient arterial constants, the preferred methodrequires that the area between the diastolic voltage V_(d) and waveformfunction f(t), or ARC, be determined. This step is shown in block 460and is preferably accomplished by determining the integral of thephotoplethysmograph voltages over the cardiac cycle using: ##EQU2##where ARC is the area between the waveform f(t) and the diastolicvoltage line V_(d), time t₀ is the time at the start of a cardiac cycle,t₁ is the time at the start of the next cardiac cycle and (t₁ -r₀) isthe cardiac cycle duration t_(d). The calculation of ARC is easilyperformed using a digital computer since the output of the A/D converter23 is a series of digital representations of the photoplethysmographsignals over time. Using the Simpson approximation to determine theintegral is particularly expedient because the digital magnitudes can bemultiplied by the sampling time between readings, then summed, to arriveat ARC. While ARC is preferably determined using integral equations,other methods of determining it are also acceptable.

Next, as shown in block 470, the photoplethylsmograph voltage, V_(m)corresponding to the mean pressure is determined from the formula

    V.sub.m =V.sub.d -(ARC/t.sub.d);

where all terms are as previously given.

With V_(m) known, the next steps, shown in block 480 and 490, are todetermine the patient's arterial constant k, solved numerically, and theratio V₀ /V_(inf) solved using either algebraic or numeric methods:##EQU3##

With the above patient arterial constant k and V₀ /V_(inf) in memory,the patient's arterial blood pressures can be determined only from thephotoplethysmograph output. This requires that various information bedetermined during a measurement period, as shown in block 320 of FIG. 3and with expanded detail in FIG. 5. Referring to FIG. 5, when arterialblood pressures are to be determined, the computer 24 monitors thephotoplethysmograph outputs to determine, at the time of measurement,the systolic voltage V_(s), the diastolic voltage V_(d), the duration ofthe cardiac cycle t_(d) and the ARC, as shown in blocks 510, 520, 530and 540, of FIG. 5 respectively. With the information V_(d), t_(d) andARC determined, the computer 24 then determines, as shown in block 550,the equivalent photoplethysmograph voltage V_(m) using the formula:

    V.sub.m =V.sub.d -(ARC/t.sub.d)

With the arterial constant k and the ratio V₀ V_(inf) determinedaccording to the flow chart of FIG. 4, and the photoplethysmographinformation determined according to the flow chart of FIG. 5, thecomputer 24 determines the patient's systolic and diastolic bloodpressures as shown in the flow chart of FIG. 6, which is a more detaileddescription of blocks 330 and 340 of FIG. 3. The most efficient methodof determining systolic and diastolic blood pressures appears to be, asshown in block 610, to first calculate the pulse pressure P_(p), usingnumerical methods, from the formula: ##EQU4## Next, the diastolic bloodpressure P_(d) is determined, as shown in block 620, using the formula##EQU5## The determination of the systolic blood pressure P_(s), is thenreadily accomplished, as shown in block 630, using the equation P_(s)=P_(d) +P_(p). While the above is the preferred method of calculatingarterial systolic and diastolic blood pressures from thephotoplethysmograph outputs, other schemes are possible.

The systolic and diastolic blood pressures are then available for outputto medical personnel as shown in block 640, in a variety of way such asby digital or analog readouts, chart recorders, voice synthesis, or asin the present embodiment on a display monitor 30. If another set ofmeasurements is desired then decision block 650 causes the flow shown inFIGS. 5 and 6 to be repeated.

The preferred embodiment described above is useful, can be readilyimplemented on a digital computer, and provides accurate and rapidmeasurements of arterial blood pressures non-invasively and in a mannersuitable for continuous measurements. However, in some patients andunder some conditions, the preferred method leads to inaccuraciesbecause of time variations in V_(inf), the equivalent receiver voltageat infinite pressure. V_(inf), in the preferred method was part of theratio V₀ /V_(inf) determined during calibration and presumed constant.The preferred embodiment can be modified to compensate for changes inV_(inf) but at the expense of additional computation difficulty andtime.

The alternative embodiment follows the same three major steps as shownin FIG. 3 for the preferred embodiment. However, the calibrationprocedure of FIG. 4 is modified to that shown in FIG. 7. Thesecalibration procedures, shown in FIG. 7 blocks 710 through 780, areidentical until V_(inf) is determined in block 790. It can be shown thatV_(inf) is determinable by the following formula:

    V.sub.inf =exp{[ln(V.sub.s)-(lnV.sub.d)exp(-kP.sub.p)]/[l-exp(-kP.sub.p)]}

With V_(inf) thus determined in block 790, V₀, the equivalent receivervoltage at zero pressure, is determined, as shown in block 799, from theformula: ##EQU6##

After the photoplethysmograph output is calibrated according to thealternative embodiment, as shown in FIG. 7, the patient constants k andV₀ are known.

According to the alternative embodiment, the data gathering stepsdepicted in FIG. 5 remain the same. However, during blood pressuredetermination, the flow diagram of FIG. 6 is modified to the proceduralsteps shown in FIG. 8. Referring now to FIG. 8, after determination ofthe pulse pressure P_(p) in block 810, in the same manner as it wasdetermined in block 610, the V_(inf) at the time of measurement isdetermined, as shown in block 820, from equation:

    V.sub.inf =exp{ln(V.sub.s)-[exp(-kP.sub.p)]lnV.sub.d ]/[l-exp(-kP.sub.p)]}

where V_(s) and V_(d) are also the values at the time of measurement.

This new V_(inf) is then used in the equation of block 830, along withthe previously stored value of V₀, to determine the diastolic pressureP_(d). This alternative embodiment reduces the effects of changes inV_(inf). The calculation of the systolic pressure P_(s), shown in Block840, and the output of the systolic and diastolic pressures, P_(d) andP_(s), respectively, as shown in block 850 are performed in the samemanner as they were in blocks 630 and 640, respectively, of FIG. 6.Likewise, the decision block 860 operates in the same manner as thedecision block 650 in FIG. 6.

The apparatus for practicing the present invention uses a modified pulseoximeter-type photoplethysmograph 4 having numerous user controls, suchas receiver 18 gain and light source 8 current settings. It outputs ananalog voltage representation of the photodiode output to ananalog-to-digital converter A/D 23 which digitizes the receiver 18output and applies it to an IBM-AT type personal computer 24 under thecontrol of software stored in a hard-disk drive. The display 30 outputis on a computer monitor. The required auxiliary blood pressureinstrument 20 readings are input by keyboard when directed by softwareprogrammed prompts. In future applications, the separatephotoplethysmograph 4, A/D converter 23, and computer 24 will probablybe replace by similar structures within a single chassis and calibrationdata will be automatically inputted by an automatic blood pressure cuff.

From the foregoing, it will be appreciated that the invention, asdescribed herein for purposes of illustration, provides an advancementin non-invasive blood pressure instruments. Although alternativeembodiments have been described herein, various modifications may bemade without departing from the spirit and scope of the presentinvention. Accordingly, the scope of the invention extends to the broadgeneral meaning of the appended claims.

We claims:
 1. A method of determining arterial blood pressures,comprising:attaching a plethysmograph to a patient so that saidplethysmograph interacts with an artery of said patient, saidplethysmograph generating an output signal having a predeterminedrelationship to a characteristic of blood in said artery; calibratingsaid plethysmograph during a calibration period by determining thepatient's actual arterial blood pressure by means other than saidplethysmograph, and then determining the value of a first arterialcharacteristic in a predetermined relationship between said firstarterial characteristic, arterial blood volume as indicated by saidplethysmograph output signal, a conversion value corresponding toarterial blood volume at infinite pressure, and said actual arterialblood pressure during said calibration period; and analyzing saidplethysmograph output signal during a measurement period to determine anarterial blood pressure corresponding to said output signal inaccordance with said predetermined relationship.
 2. The method of claim1 wherein said conversion value corresponding to arterial blood volumeat infinite pressure is determined by examining the relationship betweenarterial blood volume and arterial blood pressure, and then determiningsaid arterial blood volume at infinite pressure as the asymptotic valueof arterial blood volume in said relationship.
 3. The method of claim 1wherein said step of determining the value of said first arterialcharacteristic includes the step of expressing said first arterialcharacteristic as a logarithmic function of said plethysmograph outputsignal.
 4. The method of claim 1 wherein said step of determining thevalue of said first arterial characteristic includes the step ofexpressing said first arterial characteristic as an exponential functionof said actual arterial pressure during said calibration period.
 5. Themethod of claim 4 wherein said predetermined relationship is defined bythe formula:

    ψ=ψ.sub.inf (1--Kexp(kp))

where ψ corresponds to the arterial blood volume as indicated by saidplethysmograph output signal, ψ_(inf) is the conversion valuecorresponding to said arterial blood volume at infinite pressure, kcorresponds to said first arterial characteristic, K corresponds to asecond arterial characteristic, and P corresponds to said actualarterial pressure during said calibration period.
 6. The method of claim1, further including the steps of determining from said plethysmographoutput signal a value t_(d) corresponding to a duration of the cardiaccycle during said measurement period, a value S corresponding tosystolic pressure during said measurement period, a value Dcorresponding to diastolic pressure during said measurement period, anda value ARC corresponding to an integral with respect to time of thedifference between the plethysmograph output signal during saidmeasurement period and a value of said plethysmograph output signalcorresponding to diastolic pressure.
 7. The method of claim 1, whereinsaid step of calibrating said plethysmograph during a calibration periodincludes the steps of:determining an actual arterial systolic,diastolic, and mean blood pressures, P_(s) P_(d), and P_(m),respectively, during said calibration period by means other than saidplethysmograph; determining values V_(d) and V_(s) of saidplethysmograph output signal corresponding to respective diastolic andsystolic arterial pressures during said calibration period; determininga value V_(m) corresponding to a mean of said plethysmograph outputsignal during said calibration period; and calculating said firstarterial characteristic k from the relationship: ##EQU7## where V_(d)and V_(s) are respective values of said plethysmograph output signalcorresponding to diastolic and systolic pressures during saidcalibration period, P_(d), P_(s), and P_(m) are respective valuescorresponding to said actual arterial diastolic, systolic and meanarterial pressures during said calibration period, and k is the value ofsaid first arterial characteristic determined during said calibrationperiod.
 8. The method of claim 7, wherein said step of determining avalue V_(m) corresponding to the mean of said plethysmograph outputsignal during said calibration period is accomplished by calculatingV_(m) from the relationship:

    V.sub.m =V.sub.d -(ARC/t.sub.d)

where V_(d) is a value of said plethysmograph output signalcorresponding to diastolic pressure during said calibration period, ARCis an area between the output of said plethysmograph and V_(d), andt_(d) is a period of the output of said plethysmograph.
 9. The method ofclaim 1 further including the step of determining from saidplethysmograph output signal a value X corresponding to arterial pulsepressure during said measurement period by the steps of:determiningvalues V_(d) and V_(s) of said plethysmograph output signalcorresponding to respective diastolic and systolic arterial pressuresduring said calibration period; determining a value V_(m) correspondingto a mean of said plethysmograph output signal during said calibrationperiod; and calculating the value X corresponding to arterial pulsepressure during said measurement period from the relationship: ##EQU8##where V_(d), V_(s) and V_(m) are respective values of saidplethysmograph output signal corresponding to diastolic, systolic, andmean arterial pressures during said calibration period, and k is thevalue of said first arterial characteristic determined during saidcalibration period.
 10. The method of claim 9 wherein said step ofcalibrating said plethysmograph during said calibration period furtherincludes the steps of:determining an actual arterial systolic anddiastolic blood pressures, P_(s) and P_(d), respectively, during saidcalibration period by means other than said plethysmograph; calculatingthe ratio V_(o) /V_(inf) during said calibration period from therelationship: ##EQU9## where V_(d) and V_(s) are respective values ofsaid plethysmograph output signal corresponding to diastolic andsystolic arterial pressures during said calibration period, P_(d) andP_(s) are respective values corresponding to said actual arterialdiastolic and systolic arterial pressures during said calibrationperiod, V_(o) is a value corresponding to arterial blood volume at zeropressure, V_(inf) is a value corresponding to arterial blood volume atinfinite pressure, and k is the value of said first arterialcharacteristic determined during said calibration period.
 11. The methodof claim 10 further including the step of determining from saidplethysmograph output signal a value D corresponding to diastolicpressure during said measurement period by calculating D from therelationship: ##EQU10## where V_(d) and V_(s) are the respective valuesof said plethysmograph output signal during said calibration periodcorresponding to diastolic and systolic pressure, X is a valuecorresponding to said arterial pulse pressure during said measurementperiod determined in accordance with the method of claim 9, the ratioV_(o) /V_(inf) was determined in accordance with the method of claim 10,and k is the value of said first arterial characteristic determinedduring said calibration period.
 12. The method of claim 9, wherein saidstep of calibrating said plethysmograph during said calibration periodfurther includes the step of:determining V_(inf) at said calibrationperiod from the relationship:

    V.sub.inf =exp{[1n(V.sub.s)-(1nV.sub.d) exp(-kX)]/[1-exp(-kX)]};

determining V_(O) from the relationship: ##EQU11## where V_(d) and V_(s)are the respective values of said plethysmograph output signalcorresponding to diastolic and systolic pressure during said calibrationperiod, P_(d) and P_(s) are respective values corresponding to saidactual arterial diastolic and systolic pressures during said calibrationperiod, X is the value corresponding to said arterial pulse pressureduring said measurement period determined in accordance with the methodof claim 11 V_(o) is a value corresponding to arterial blood volume atzero pressure, V_(inf) is the value corresponding to arterial bloodvolume at infinite pressure, and k is the value of said first arterialcharacteristic determined during said calibration period.
 13. The methodof claim 12 further including the step of determining from saidplethysmograph output signal a value D corresponding to diastolicpressure during said measurement period by calculating D from therelationship: ##EQU12## where V_(d) and V_(s) are the respective valuesof said plethysmograph output signal during said calibration periodcorresponding to diastolic and systolic pressure, X is the valuecorresponding to said arterial pulse pressure during said measurementperiod determined in accordance with the method of claim 9, V_(o) andV_(inf) were determined in accordance with the method of claim 12, and kis the value of said first arterial characteristic determined duringsaid calibration period.